Flashing in building construction
Flashing is a sheet of thin, impervious material used to prevent water penetration or seepage into a building and to direct the flow of moisture in walls. Flashing is particularly important at junctions such as roof hips and valleys, joints between roofs and vertical walls, roof intersections or projections such as chimneys, dormers, vent pipes, window openings, and so on.
There are two categories of flashing, exposed and embedded. Exposed flashings are partially exposed to the outside and usually made of a sheet metal, while embedded (or concealed) flashing can be metal or non-metal and are typically used at the base of cavity walls, above openings, at sills and shelf angles (a structural steel member that supports and transfers the brick’s dead load back to the building frame), and under copings.
There are different terms given to flashing, often determined by where they are used or their shape.
- Roof flashing: Installed around projections or intersections to direct water from seams or joints.
- Wall flashing: Typically embedded in wall interruptions such as windows, either to prevent water ingress or direct water that has entered back outside.
- Sill flashing: Concealed under windows or door thresholds to prevent water ingress.
- Channel flashing: U-shaped channel used where the edge of a tile roof meets a wall.
- Through wall flashing: Directs water to weep holes by spanning the thickness of the wall.
- Cap flashing: Above windows and doors.
- Drip edge: A metal projection often found at roof edges.
- Stepped flashing: Flashing materials that overlap each other in steps where a junction is on a slope, such as around chimney stacks on pitched roofs.
- Kickout flashing: The lowermost step flashing at the bottom of roof/wall intersection, used to deflect water away from the wall.
- Pipe flashing: Used where roofs are penetrated by pipes.
- Valley flashing: Where two intersecting roof planes form a valley in which water may pool.
Step flashing allows a roof to abut brickwork, instance when a lower height roof meets a wall, a roof meets a chimney, or a conservatory roof meets a house wall. Step flashing offers far better protection from leaks than continuous flashing, because even if a single piece of step flashing fails, the water hits the next lower piece, which directs the water onto the shingle and drains down the roof.
There are many materials that can be used for flashing; the important thing is that they do not degrade from coming into contact with incompatible materials and that they are impermeable to water. The most common flashing materials are metal, plastic, or rubber.
Lead is traditionally used as a flashing material around upstands in roofs, such as chimneys because of its durability. The minimum specification that should be used is Code 4 lead (1.8 mm) which is both malleable and resistant to fatigue splits that could be caused by expansion and contraction. Code 3 (1.32 mm) is thinner, lighter and cheaper, and is commonly used for lead soakers which are the concealed waterproof sections positioned underneath slates and plain tiles.
Other materials that can be used as metal flashings include aluminium which is also durable and relatively inexpensive. However, aluminium will corrode if left in contact with pressure-treated wood, and also wet mortar and plaster.
Copper is malleable, strong and has a high resistance to the corrosive effects of mortars and wood-preservatives. Cold rolled copper is recommended rather than soft copper as it offers more resistance to expansion and contraction stresses, although soft copper is useful where there are complex roof shapes.
Soft zinc is a very malleable material which makes it suitable for complex roof shapes.
There are also arrangements of metals that can be coated for use as flashings, such as:
- Galvanized steel, which is inexpensive but lacks the durability of other metals.
- Lead-coated copper.
- Anodized aluminium.
- Terne-coated copper.
- Galvalume (aluminium-zinc alloy coated sheet steel).
- Polyvinylidene fluoride.
Neither copper nor lead can be used in contact with or above aluminium, zinc or coated steel without an increased risk of premature corrosion. Coastal areas face the risk of accelerated corrosion due to salt spray and so stainless steel, copper or coated aluminium are often recommended.
Also known as flexible flashing materials, these include rubberised asphalt, butyl rubber, PVC and acrylic. They have different chemical compatibilities, resistance to ultraviolet light exposure, application temperature ranges and so on. Adhesive-backed materials are easily installed but consideration should be given to their long-term water resistance and most should not be left permanently exposed to the weather unless laminated with a top surface of aluminium foil.
 Related articles on Designing Buildings Wiki
- Cavity tray.
- Cold bridge.
- Damp proof course.
- Flat roof defects.
- Lead in construction.
- Penetrating damp.
- Rising damp.
- Roof insulation.
- Roof tiles.
- Roofing defects.
- Vapour barrier.
- Wall ties.
- Weep hole.
 External references
- Old House Web - Flashing: What is it and why is it important?
- ‘Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann, (2007)
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